Parameters

escape-expression The escape character to be used in the match. The default is the backslash character (\).

Remarks

The REGEXP search condition matches a whole string, not a substring. To match on a substring with the string, enclose the
string in wildcard percent signs (%pattern%). For example, SELECT ... WHERE Description REGEXP 'car' matches only car, not sportscar. However, SELECT ... WHERE Description REGEXP '%car' matches car, as well as sportscar, and any string that ends with car. Alternatively, you can rewrite your query to make use
the REGEXP_SUBSTR function, which is designed to search for substrings within a string.

When matching against a sub-character class in an empty larger character class, remember to include the outer square brackets,
as well as the square brackets for the sub-character class (for example, expression REGEXP '[[:digit:]]'). For more on sub-character class matching, see Regular expressions: Special sub-character classes.

How comparisons are performed

Comparisons are performed character-by-character, unlike the equivalence (=) operator and other operators where the comparison
is done string-by-string. For example, when a comparison is done in a UCA collation (CHAR or NCHAR with the collation set
to UCA), 'Æ'='AE' is true, but 'Æ' REGEXP 'AE' is false.

For a character-by-character comparison to match, each single character in the expression being searched must match a single
character (using the collation's character equivalence), or a wildcard in the REGEXP pattern.

National character (NCHAR) support

REGEXP search conditions can be used to compare CHAR and NCHAR strings. In this case, character set conversion is performed
so that the comparison is done using a common data type. Then, a character-by-character comparison (or code point by code
point, in the case of UTF8 databases) is performed. See Comparisons between CHAR and NCHAR.

You can specify expression or pattern as an NCHAR string literal by prefixing the quoted value with N (for example, expression REGEXP N'pattern'). You can also use the CAST function to cast the pattern to CHAR or NCHAR (for example, expression REGEXP CAST(pattern AS datatype).